In many aspects elongated members of wind turbines are exposed to stress that causes strain. For example, the tower of a wind turbine as well as the wind turbine rotor blades may be exposed by strong wind which causes a strain on the tower and the rotor blades. The load on the rotor blades and/or the tower may be reduced by a variation of the blade's pitch. This can be realised by individual pitch controllers in general or by a cyclic pitch controller.
An effective pitch control, for example for reducing the loads acting on the rotor, needs information about the load acting on the blade root which can be extracted from a determination of the rotor blade deflection. Optical fibres in the blades are typical sensors for a reliable and long term measurement of the rotor loads. Such systems are rather expensive.
In U.S. Pat. No. 7,059,822 B2 a method for determining a rotor blade deflection is disclosed wherein a rotor blade is coupled with a hub. The rotor blade includes a beam with a first end coupled to a baffle inside the rotor blade, while the second end of the beam is located adjacent to the hub and is used for measuring the deflection of the beam by means of at least one sensor located in the hub. The beam is located near the centreline of the rotor blade. The determination of the deflection of the blade is based on the movement of the beam which correlates with the blade deflection.
In WO 03/029750 A1 a sensor construction for measuring the bending of a construction element is disclosed. It comprises a rod element positioned offset relative to the deformation neutral line/surface for the bending to be measured. A first end of said rod element is firmly connected to the construction element and a second end of said rod element is connected to the construction element. The second end connection provides a movability in the axial direction of the rod element. The measurement is performed by measuring the position of the second end of the rod element relative to the construction element.
In US 2006/0000269 A1 a method for determining rotor blade deflection is described, wherein a rotor blade is coupled to a hub. A first end of the a beam is coupled to the rotor blade. A second end of the beam is positioned adjacent the hub. The deflection of the beam is measured by use of at least one sensor. The deflection of the blade is determined based on the deflection of the beam.
In US 2006/0201257 A1 a gas turbine blade fatigue life evaluating method for qualitatively evaluating the fatigue life of a turbine blade is described. The gas turbine blade is to be within its fatigue life if the creep elongation strain in the longitudinal direction of the turbine blade is less than 0.5% of an initial length. Moreover, a gas turbine blade creep elongation strain measuring apparatus which comprises a first fixed end, a second fixed end and a dial gauge. A dimension in the longitudinal direction is stamped on the surface of a turbine blade.